The higher activity observed for the weaker LA extends
to other aromatic aldehyde substrates (Table 1). Benzalde-
hyde itself is a more active substrate, but at both -44 and
-78 °C PhB(C6F5)2 is a more effective catalyst than
B(C6F5)3.8 The same trend exists for p-chlorobenzaldehyde,
while the difference is less pronounced for the highly
activated substrate p-nitrobenzaldehyde at -78 °C. This latter
observation is likely a reflection of the effect of the para
substituent on the thermodynamic stability of the ion pairs
2 and suggests that the balance between path a or b is also
affected by the nature of the substrate.
Thus, although B(C6F5)3 is clearly a stronger LA than
PhB(C6F5)2,6 this leads not to higher activity as an allylation
catalyst but rather to greater thermodynamic stability of
the product of the first C-C bond forming step in the
reaction, 2a. The ion pair 2b, on the other hand, does not
fall into as deep a well and the equilibrium is more biased
toward product 4 and 1b, which in the presence of excess
allyltin reagent is rapidly allylated, shunting the reaction
toward path b. However, while the data for the equilibria of
Scheme 2 provide a partial explanation for the observed
higher activity for the weaker Lewis acid PhB(C6F5)2,
clearly there are ill-defined kinetic factors contributing to
the higher rate of transfer of the alkoxy group from boron
to tin in 2b as opposed to 2a. Possibly, the substitution
of one C6F5 group for a less electron-withdrawing C6H5
renders the alkoxy group significantly more nucleophilic
in the phenyl-substituted borate anion, lowering the bar-
rier to product-forming transfer to tin. Given that the in-
timate mechanisms of these equilibria likely involve mul-
tiple steps, the factors affecting the observed rates are
complex.
In summary, unlike B(C6F5)3, the weaker LA PhB(C6F5)2
likely functions as a true LA catalyst for the allylstanna-
tion reaction and is in practical terms much more effective
than either B(C6F5)3 or [Bu3Sn(L)]+. In effect, B(C6F5)3
is partially buffered to the strength of [Bu3Sn(L)]+, and
as previously determined,4 a significant portion of the
catalysis occurs via path a for this borane. PhB(C6F5)2 is
strong enough to promote allylation but weak enough to more
effectively transfer the OR group to tin to complete the
reaction via path b. This concept can only be extended so
far, however; the substantially weaker LA BPh3 (Child’s
Lewis acidity ) 0.06) is not an active catalyst for the
allylstannation of benzaldehyde at or below -44 °C, so a
threshold borane Lewis acidity is required in order to initiate
this reaction.9
Table 1. Lewis Acid Catalyzed Allylstannation of
Benzaldehydea
entry
catalyst
B(C6F5)3
PhB(C6F5)2 o-MeO-C6H4 -44
Ar
T (°C) time (min) convn (%)b
1
2
o-MeO-C6H4 -44
75
75
21
100
95
100
62
96
38
100
95
100
3
4
5
6
7
8
9
B(C6F5)3
PhB(C6F5)2 C6H5
B(C6F5)3 C6H5
PhB(C6F5)2 C6H5
C6H5
-44
-44
-78
-78
-78
-78
60
40
600
600
300
300
70
B(C6F5)3
p-Cl-C6H4
PhB(C6F5)2 p-Cl-C6H4
B(C6F5)3
p-NO2-C6H4 -78
10
PhB(C6F5)2 p-NO2-C6H4 -78
70
a Conditions: CH2Cl2, 0.1 M in ArCHO, 0.5 mmol ArCHO, 0.55 mmol
allylSnBu3, 5.0 mol % catalyst loading. b Conversion of ArCHO by GC
after quenching reaction aliquot into H2O.
counterpart,6 is a significantly more active allylation catalyst
than B(C6F5)3 toward o-anisaldehyde (Table 1, entries 1 and
2).
To probe the origin of this unusual observation, the
thermoynamic data for the equilibrium involving ion pair
2b was acquired in a fashion analogous to that described
above for 2a (Scheme 2). As with the B(C6F5)3-promoted
reaction, all of the allylSnBu3 was rapidly and irreversibly
consumed in a C-C bond forming reaction with borane-
activated aldehyde. Equilibrium was reached after ∼5 h at
-80 °C. The only signals in the 119Sn NMR spectrum of the
equilibrium mixture are of stannyl ether 4 at 110 ppm and a
signal identical to that reported for the bis-aldehyde ligated
tributyltin cation [(o-anisaldehyde)2SnBu3]+ at 92 ppm.4 The
19F NMR spectrum indicates the presence of only two
fluorine-containing species; the borane-aldehyde adduct 1b
and a second set of signals due to the alkoxy borate species
2b.7 Finally, van’t Hoff analysis of this equilibrium (Figure
1, open triangles) yields a ∆S value of 18.3(1) eu and a ∆H
of 5.70(5) kcal mol-1. Not surprisingly, the ∆S values for
these two equilibria are quite similar, but the ∆∆H of 1.51-
(5) kcal mol-1 indicates that the ion pair 2b is less
enthalpically stable than the fully fluorinated alkoxyborate
2a. The least-squares analysis of the van’t Hoff plots allows
estimation of Keq values at one temperature for direct
comparison of the two boranes. At 193 K, Keq values are
estimated to be 3.0 × 10-3 for PhB(C6F5)2 and 6.0 × 10-5
for B(C6F5)3.
The ability to “turn on” path b via modification of Lewis
acidity has significant implications for development of this
chemistry, since chiral boranes can be expected to be
effective for asymmetric allylation procedures by this mech-
(5) Deck, P. A.; Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998,
120, 1772.
(6) (a) PhB(C6F5)2 exhibits a Child’s Lewis acidity6b (0.54) lower than
that of B(C6F5)3 (0.68). In addition to significantly weaker Child’s Lewis
acidity, in a competition experiment between B(C6F5)3 (1 equiv) and
PhB(C6F5)2 (1 equiv) for PhCHO (0.9 equiv) in CD2Cl2, only the B(C6F5)3-
PhCHO adduct 1a was observed by 19F NMR, confirming the higher
Lewis acidity of B(C6F5)3 for PhCHO (see Supporting Information).
(b) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, 60,
801.
(8) (a) At the same temperatures and times, use of separately prepared8b
stannylinium cation [Bu3Sn]+[B(C6F5)4]- as a catalyst gives conversions
of 40% at -44°C and 9% at -78°C. (b) Lambert, J. B.; Kuhlmann, B. J.
Chem. Soc., Chem. Commun. 1992, 931.
(9) (a) Unfortunately, the final member of this family of boranes, namely,
Ph2B(C6F5) is prone to redistribution,9b such that small amounts of PhB-
(C6F5)2 are always present, skewing comparisons between the two. (b)
Bradley, D. C.; Harding, I. S., Keefe, A. D.; Motevalli, M.; Zheng, D. H.
J. Chem. Soc., Dalton Trans. 1996, 3931.
(7) The spectroscopic signature of the alkoxyborate anion in 2b was
identical to that of its [NBu4]+ salt, generated separately (see Supporting
Information for details).
Org. Lett., Vol. 5, No. 16, 2003
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